Biomedical electrodes are used in a variety of applications and are configured to operate according to the size, type, and direction of current flowing into or out of a body of a patient.
Dispersive electrodes are used in electrosurgery. In modem surgical practice there are many times when electrosurgery is more preferable than the use of the traditional scalpel. In electrosurgery, cutting is performed by an intense electrical current passing through a cutting electrode. The surgeon directs this current to exactly where cutting is required by wielding the cutting electrode, which because of its cylindrical shape and the way it is held in the hand is commonly called an "electrosurgical pencil". By activating controls which change the characteristics of the electrical current being sent to the pencil by an electrosurgical generator, the surgeon can use the pencil either to cut or to coagulate areas of bleeding. This makes electrosurgery particularly convenient when surgery requiring extra control of blood loss is being performed. Because of concerns to minimize the transmissions of blood-borne illnesses between health care patients and health care providers, in both directions, electrosurgery is becoming increasingly important.
In electrosurgery, as in all situations where electrical current is flowing, a complete circuit must be provided to and from the current source. In this case, the current that enters the body at the pencil must leave it in another place and return to the generator. It will readily be appreciated that when current enough to deliberately cut is brought to the body of a patient in one place, great care must be taken that unintentional damage is not also done to the patient at the location where that current is leaving the body. The task of collecting the return current safely is performed by a dispersive electrode.
A dispersive electrode performs this task by providing a large surface area through which the current can pass; the same current which was at cutting intensity when focused at the small surface area at the tip of the pencil is relatively harmless, with the goal of being painless to the patient, when spread out over the large surface area of the dispersive electrode.
Unfortunately, any geometry of the large surface area has an edge and perhaps distinct corners or junctions where "edge effects" caused by increased current density at those locations can have a maximum temperature rise during usage by the patient making such dispersive electrode or cardiac stimulating electrode most uncomfortable to the patient.
Use of dispersive biomedical electrodes must account for the size of the electrode relative to the location of edges where current density is highest and where discomfort sensed in adjoining tissue of the patient caused by such current density.
The same difficulties concerning edge effect also are present in cardiac stimulating electrodes, such as those used for defibrillation, external pacing, or cardioversion. For a patient already in some discomfort or ill health, pain sensed by the very medical device intended to treat the patient is disconcerting at best to the patient.